A coal combustion flue gas or flue gas generated when burning heavy oil sometimes contains metallic mercury (Hg0) in addition to soot and dust, sulfur oxides (SOx), and nitrogen oxides (NOx). In recent years, various methods and devices for treating this metallic mercury have been devised in combination with a denitration device for reducing NOx and a wet desulfurization device using an alkali absorbent as an SOx absorbent.
As a method for treating metallic mercury in a flue gas, there has been suggested a system in which an ammonium (NH3) solution is sprayed in a flue gas duct on an upstream side of a high-temperature denitration device so as to perform reduction and denitration; an oxidation auxiliary agent such as a hydrochloric acid (HCl) solution is also sprayed so as to oxidize (chlorinate) mercury on a denitration catalyst to obtain a water-soluble mercury chloride; and the amount of mercury is then reduced by a wet desulfurization device disposed on a downstream side (for example, see Patent Literature 1).
Moreover, as a method for supplying HCl, there has been a method in which a hydrochloric acid (HCl) solution is vaporized using a hydrogen chloride (HCl) carburetor to obtain a hydrogen chloride (HCl) gas; and after adjusting it to a mixed gas containing HCl with a predetermined concentration, the mixed gas is dispersed into a flue gas duct so as to be sprayed evenly into a flue gas containing mercury (for example, see Patent Literature 2).
Moreover, as another method for supplying HCl, there has been a method in which powdered ammonium chloride (NH4Cl) is added in a flue gas duct on an upstream side of a denitration device; NH4Cl is sublimed by a high ambient temperature of a flue gas, so that HCl and ammonia (NH3) are gasified respectively; and the gasified HCl gas and NH3 gas are mixed with the flue gas (for example, see Patent Literature 3).
With the methods for treating metallic mercury in a flue gas as described above, when a hydrochloric acid solution is used, there is a problem that great care and cost are required in its transportation, handling, and the like since hydrochloric acid is a dangerous substance. Moreover, when the HCl carburetor is used, steam or the like is required as a heat source. Thus, there is a problem that costs for the facility, operation, maintenance, and the like of the HCl carburetor and the like are required. Furthermore, when NH4Cl powders are used, a particle diameter thereof needs to be reduced for the dispersion thereof. Thus, there is a problem that the handling thereof is difficult and control for the sprayed amount thereof is not easy.